Both are inactive for Sunday night's game because of hamstring injuries.
Jonathan Stewart, who ran for 37 yards and a touchdown last week against Detroit, will make his second straight start for Williams. Either Jason Avant or Brenton Bersin is expected to start for Cotchery.
Panthers coach Ron Rivera had expressed optimism Friday over Williams' chances of playing.
Pittsburgh's inactives include quarterback Landry Jones, wide receiver Martavis Bryant running back Dri Archer, offensive linemen Ramon Foster and Wesley Johnson, nose tackle Daniel McCullers and cornerback B.W. Webb.
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